Fast controlled output buffer

ABSTRACT

An output buffer switch-on control circuit includes several transistors and a discharge current control circuit. A first transistor has a first terminal connected to an internal voltage line and is controlled by an output data source. A second transistor has a first terminal connected to the internal voltage line and is controlled by a second terminal of the first transistor. The second transistor also has a second terminal connected to a first terminal of an output capacitor. A third transistor is controlled by the output data source and has a first terminal connected to a common voltage. A fourth transistor is digitally controlled and has a first terminal connected to the second terminal of the second transistor. The fourth transistor also has a second terminal connected to the common voltage. The discharge current control circuit is preferably actively-controlled and is connected between a second terminal of the first transistor and a second terminal of the third transistor. The discharge current control circuit preferably includes a discharge resistor and a mirrored current transistor feedback controlled by an output capacitor.

PRIORITY

This application claims the benefit of priority to Italian patent application number T02002A000811, filed on Sep. 18, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to output buffers, and particularly to controlling an internal VDDQ reference voltage around a target value when short capacitor charge times are desired.

2. Discussion of the Related Art

In integrated devices, an internal power source may be viewed as an RLC model (resistance-inductance-capacitance) between an external pin and integrated transistors. A schematic representation of a simplified circuit according to this model is illustrated at FIG. 1. FIG. 1 shows an external voltage or VDDQ_(GEN) (or VDDQ_GEN in FIG. 1) connected through inductance L to resistors R_(i), . . . , R_(n) and capacitors C_(i), . . . , C_(n), wherein the capacitors C_(i), . . . , C_(n) power the internal voltages or VDDQ_(internal) (or INTERNAL_VDDQ) in FIG. 1). With this model in mind, problems are observed as being caused by inductance and resistance when it is desired to charge a relatively large capacitance in a very short time, i.e., on the order of nanoseconds (ns).

In these cases, a large amount of current is flowed involving a significant drop on the resistance. If left uncontrolled, fluctuations of the power current dropped at the resistance induce drops or overshoots of the values of the internal power VDDQ_(internal) (by the inductance).

These drops and overshoots may generate variations of the values of VDDQ_(internal) resulting in undesirable consequences. For example, an uncontrolled drop of VDDQ_(internal) below a trigger point voltage may turn off p-mos transistors that take VDDQ as a high reference voltage. Also, an eventual VDDQ_(internal) drop below the trigger point voltage may slow output switching for those transistors that have V_(gate) equal to zero. This undesired effect may occur due to delays associated with waiting for the VDDQ to recover before detecting the V_(out) logic value to be “1”.

The charging and discharging of the output data pin, i.e., characterized by a relatively large capacitance, is one of the situations wherein this effect may produce significant undesirable effects. To prevent these effects, current control may be provided when the output buffers are switching on.

The control of the VDDQ absorbed current may be achieved by different techniques. One technique is controlling the p-mos buffer turn on. The buffer elements are not switched on in digital mode, as is typical with traditional architectures, but their VGS absolute values rise in time with a pending control.

FIG. 2, e.g., schematically illustrates a conventional architecture. The conventional architecture of FIG. 2 has VDDQ_(internal) connected to the p-mos (P4) transistors M₀ and M₃. The p-mos transistor M₃ is connected to n-mos (N) transistor M₂. The p-mos transistor M₀ is connected to n-mos (N) transistor M₁. The n-mos transistors M₁ and M₂ are each also connected to ground. An input control signal data_(out) (or OUT_DATA in FIG. 2) controls each of the p-mos transistor M₃ and the n-mos transistor M₂. The output of the p-mos transistor M₃ controls each of p-mos transistor M₀ and n-mos transistor M₁. The output of the p-mos transistor M₀ is connected to capacitor C_(out).

The discharge current may be controlled, as in the circuit of FIG. 2, by the turning to ground of the gate of the p-mos transistor M₀ when data_(out) is low. In this way, current absorbed by the out buffer, when the output data changes from “0” to “1”, has a continuous profile in the time without abrupt variations.

RECOGNIZED IN THE INVENTION

It is recognized in the present invention that a determination of the value of the gate discharge current may be achieved by modifying the circuit of FIG. 2 in different ways according to FIG. 3. These modifications each include insertion of a device D between the output of p-mos transistor M₆ corresponding to p-mos transistor M₃ of FIG. 2, and n-mos transistor M₄, corresponding to n-mos transistor M₂ of FIG. 2. The output of p-mos transistor M₆ still controls p-mos transistor M₇, corresponding to p-mos transistor M₀ of FIG. 2. The n-mos transistor M₅, corresponding to n-mos transistor M₁ of FIG. 2, is now controlled by the digital N-control. Specific modifications include adding a discharge resistor as the device D, as in the circuit of FIG. 4, or adding a mirrored current transistor as the device D, as in the circuit of FIG. 5.

Referring to FIG. 4, the discharge resistor (RP) R₁ may be inserted between the output of the p-mos transistor M₁₀ corresponding to the p-mos transistor M₃ of FIG. 2, and the n-mos transistor M₈ corresponding to the n-mos transistor M₂ of FIG. 2. The output of p-mos transistor M₁₀ would still control the p-mos (P4) transistor M₁₁ corresponding to the p-mos transistor M₀ of FIG. 2. The n-mos transistor (N) M₉ of the circuit of FIG. 4, and corresponding to the n-mos transistor M₁ of FIG. 2, would be controlled by digital N-control, rather than by the output of p-mos transistor M₁₀ as in the circuit of FIG. 2.

Referring to FIG. 5, the mirrored current transistor M₁₆ may be inserted between the output of the p-mos transistor M₁₄ corresponding to the p-mos transistor M₃ of FIG. 2, and the n-mos transistor M₁₂ corresponding to the n-mos transistor M₂ of FIG. 2. The mirrored current transistor M₁₆ is controlled by I_(mirror) (or I_MIRROR in FIG. 5). The output of p-mos transistor M₁₄ would still control the p-mos (P4) transistor M₁₅ corresponding to the p-mos transistor M₀ of FIG. 2. The n-mos transistor (N) M₁₃ of the circuit of FIG. 5 would be controlled by digital N-control, rather than by the output of p-mos transistor M₁₄ as in the circuit of FIG. 2.

The solutions described above with reference to FIGS. 4 and 5 may solve the problem of VDDQ drop, because the buffer turn on is controlled. However, these solutions are not preferred herein for avoiding overshoot of VDDQ when C_(out) is charged and the current goes to zero. These solutions do not involve an active control because the resistance value of R₁ or mirrored current value of M₁₆ are determined at the time the circuit is designed and are not later modifiable. It is desired to have an active control which prevents the VDDQ drop and overshoot problems described above.

SUMMARY OF THE INVENTION

In view of the above, an output buffer switch-on control is provided for avoiding internal VDDQ drop and overshoot with a limited circuital overhead. Eventual VDDQ variations are automatically corrected by active controlling implemented by an output voltage feedback arrangement.

A particularly preferred output buffer switch-on control circuit includes at least four transistors. The first transistor has a first terminal connected to an internal voltage line and is controlled by an output data source. The second transistor has a first terminal connected to the internal voltage line and is controlled by a second terminal of the first transistor. The second transistor also has a second terminal connected to a first terminal of an output capacitor. The third transistor is controlled by the output data source and has a first terminal connected to a common voltage. The fourth transistor is digitally controlled and has a first terminal connected to the second terminal of the second transistor and has a second terminal connected to the common voltage. The switch-on control circuit further includes a discharge current control circuit connected between a second terminal of the first transistor and a second terminal of the third transistor. The discharge current control circuit is advantageously preferably actively-controlled.

The discharge current control circuit preferably includes a discharge resistor and a mirrored current transistor. The mirrored current transistor is preferably controlled by a connection between the second terminal of the second transistor and the first terminal of the fourth transistor. The mirrored current transistor preferably includes a first terminal connected to the second terminal of the first transistor and preferably also includes a second terminal connected to the discharge resistor. The discharge resistor is preferably connected between the mirrored current transistor and the third transistor. The first and second transistors preferably comprise p-type MOSFETS, and the third and fourth transistors comprise n-type MOSFETS. A second terminal of the output capacitor is preferably connected to the common voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a schematic representation of a simplified RLC model illustrating an output buffer between an external pin and transistors of an integrated device.

FIG. 2 schematically illustrates a conventional current control for the switching on of output buffers.

FIG. 3 schematically illustrates a modified current control for the switching on of output buffers including a discharge current control device.

FIG. 4 schematically illustrates a discharge resistor as an example of the discharge current control device of FIG. 3.

FIG. 5 schematically illustrates a mirrored current transistor as another example of the discharge current control device of FIG. 3.

FIG. 6 schematically illustrates a drop controller transistor and discharge resistor combination with output voltage feedback according to a preferred embodiment.

FIG. 7 shows comparative simulation plots of VDDQ voltage versus time for a switch-on circuit with conventional discharge current control and for a switch-on circuit with discharge current control according to a preferred embodiment, along with a plot of the simulated V_(out) for the output capacitor used for generating the VDDQ plots.

FIG. 8 shows comparative plots of VDDQ voltage versus time for a switch-on circuit with conventional discharge current control and for a switch-on circuit with discharge current control according to a preferred embodiment, along with comparative plots of V_(out) for the output capacitor also for each of a switch-on circuit with conventional discharge current control and for a switch-on circuit with discharge current control according to a preferred embodiment.

INCORPORATION BY REFERENCE

The above sections entitled, “BACKGROUND”, “RECOGNIZED IN THE INVENTION”, “SUMMARY OF THE INVENTION”, and “BRIEF DESCRIPTION OF THE DRAWINGS” are each hereby incorporated by reference into the below section entitled, “DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT”, as disclosing alternative embodiments of elements or features of the preferred embodiment not otherwise set forth in detail below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The output buffer switch-on control circuit of the preferred embodiment includes a discharge current control circuit which is preferably actively-controlled. The preferred discharge current control circuit solves the VDDQ overshoot problem described in the background. The control of the VDDQ drop is preferably active. Preferably, a p-mos transistor is inserted into the gate discharge path. This transistor is controlled by the V_(out) voltage (or C_(out) in FIG. 6).

Referring to FIG. 6, a first p-mos transistor M₁₉ has a first terminal connected to VDDQ_(internal) The first p-mos transistor M₁₉ is controlled by data_(out), as shown. A second p-mos transistor M₂₀ has a first terminal also connected to VDDQ_(internal) The second p-mos transistor M₂₀ is controlled by connection to the second terminal of the first p-mos transistor M₁₉. A first n-mos transistor M₁₇ is also controlled by data_(out), and has a first terminal connected to a common voltage, such as ground. A second n-mos transistor M₁₈ is digital N-controlled, has a first terminal connected to the second terminal of the second p-mos transistor M₂₀, and has a second terminal connected the common voltage.

The preferred discharge current control circuit includes a mirrored current, preferably of p-mos type, transistor M₂₁ that is feedback controlled by V_(out) (or C_(out) in FIG. 6). The second terminal of the second p-mos transistor M₂₀ and first terminal of the second n-mos transistor M₁₈ are each also preferably connected to V_(out) (or C_(out)), and thus also to the gate of the mirrored current p-mos transistor M₂₁. The preferred discharge current control circuit further preferably includes a discharge resistor R₅. The mirrored current transistor M₂₁ has a first terminal connected to the second terminal of the first p-mos transistor M₁₉ which controls the second p-mos transistor M₂₀. The second terminal of the mirrored current transistor M₂₁ is connected to the discharge resistor R₅. The discharge resistor R₅ is, in turn, connected between the mirrored current transistor M₂₁ and the second terminal of the first n-mos transistor M₁₇.

During turn on, the buffer gate discharge current is defined by the resistor “R” and the p-mos “P” resistance, as shown in respective circles in FIG. 6. An eventual VDDQ drop is immediately stopped by the resistance of the p-mos transistor M₂₁ which rises with the value of V_(out) (or C_(out)). The drop control is “active”, because it depends on the value of V_(out) (or C_(out)), which rises in time. The system is able to autorecover these VDDQ drop problems.

Moreover, the rise of V_(out) (C_(out)) induces a proportional turn off of the mirrored current transistor M₂₁, or p-mos “P” of FIG. 6, which “brakes” the switching-on of the output buffer. In this way, the current which charges the C_(out) output capacitor decreases in the time with a limitation on the VDDQ overshoot value. This provides a remarkable reduction of voltage stress induced in the transistors (these stresses may otherwise cause damage in low voltage devices).

In addition, the output buffer switch-on control circuit according to the preferred embodiment generally depends on the RLC value of VDDQ_(out) according to the model schematically illustrated at FIG. 1. In the device of the preferred embodiment, the switch-on technique may be varied by employing a group of fuses to adapt switch-on circuit to real requirements of a physical device.

Results

The introduction of the current mirrored transistor M₂₁, or p-mos “P” transistor of FIG. 6, into the gate discharge path of the output buffer provides advantageous control of VDDQ variations. FIG. 7 shows comparative simulation plots of VDDQ voltage versus time. Plot A of FIG. 7 shows a VDDQ plot for a switch-on circuit with conventional discharge current control, e.g., such as that described above with reference to FIG. 2. Plot B of FIG. 7 shows a VDDQ plot for a switch-on circuit with discharge current control according to a preferred embodiment, e.g., such as that described herein with reference to FIG. 6. Plot C of FIG. 7 shows a plot of a digital signal used as an enable command for changing the value of the output that was used in generating the simulation plots A and B. For the simulations of plots A and B of FIG. 7, the capacitance of the C_(out) capacitor was 50 pF, and external VDDQ was 2.2 V, and the temperature was T=−40° C.

As may be clearly observed from a comparison of plots A and B, the voltage of plot B according to the circuit of the preferred embodiment exhibits greater stability and reduced fluctuations than the voltage of plot A according to the conventional circuit. The VDDQ drop has the same value, i.e., from 2.2 to 1.4, in both plot A and plot B. However, the p-mos controller induces an evident decrement in the duration of the VDDQ undershoot time. An analogue improvement is visible for the overshoot control. In fact, plot B according to the circuit of the preferred embodiment is practically free of VDDQ overshoot, while plot A according to the conventional circuit exhibits a very large overshoot.

FIG. 8 shows further comparative plots of VDDQ voltage versus time. Plot A shows voltage versus time for a switch-on circuit with conventional discharge current control. Plot B shows voltage versus time for a switch-on circuit with discharge current control according to a preferred embodiment. FIG. 8 also shows comparative plots of V_(out) for the output capacitor. Plot C shows V_(out) versus time for a switch-on circuit with conventional discharge current control. Plot D shows V_(out) versus time for a switch-on circuit with discharge current control according to a preferred embodiment. For the plots A-D of FIG. 8, the capacitance of the C_(out) capacitor was 50 pF, and external VDDQ was 1.5 V, and the temperature was T=125° C. As with the plots of FIG. 7, a digital signal was used as an enable command for changing the value of the output. Dimensions of the uncontrolled buffer are those limiting VDDQ drop.

FIG. 8 illustrates that with a comparable VDDQ drop and overshoot for the conventional case (plot A) and for the circuit of the preferred embodiment (plot B), controlling the V_(out) rise is quicker the p-mos discharge gate controller of the preferred embodiment. Advantageously, the time to charge C_(out) to the trigger point voltage (i.e., VDDQ/2) is shown as being about 20% faster for the circuit of the preferred embodiment compared with the conventional circuit.

Those skilled in the art will appreciate that the just-disclosed preferred embodiments are subject to numerous adaptations and modifications without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope and spirit of the invention, the invention may be practiced other than as specifically described above. The scope of the invention is thus not limited by the particular embodiments described above. Instead, the scope of the present invention is understood to be encompassed by the language of the claims that follow, and structural and functional equivalents thereof. 

We claim:
 1. An output buffer switch-on control circuit, comprising: a first transistor having a first terminal connected to an internal voltage line and controlled by an output data source; a second transistor having a first terminal connected to the internal voltage line and controlled by a second terminal of the first transistor and having a second terminal connected to an output capacitor; a third transistor controlled by the output data source and having a first terminal connected to a common voltage; a fourth transistor that is digitally controlled and has a first terminal connected to the second terminal of the second transistor and has a second terminal connected to the common voltage; and a discharge current control circuit connected between a second terminal of the first transistor and a second terminal of the third transistor.
 2. The switch-on control circuit of claim 1, wherein the discharge current control circuit includes a discharge resistor.
 3. The switch-on control circuit of claim 1, wherein the discharge current control circuit includes a mirrored current transistor.
 4. The switch-on control circuit of claim 3, wherein the discharge current control circuit further includes a discharge resistor.
 5. The switch-on control circuit of claim 4, wherein the a mirrored current transistor includes a first terminal connected to the second terminal of the first transistor and includes a second terminal connected to the discharge resistor, the discharge resistor being connected between the mirrored current transistor and the third transistor.
 6. An output buffer switch-on control circuit, comprising: a first transistor having a first terminal connected to an internal voltage line and controlled by an output data source; a second transistor having a first terminal connected to the internal voltage line and controlled by a second terminal of the first transistor and having a second terminal connected to a first terminal of an output capacitor; a third transistor controlled by the output data source and having a first terminal connected to a common voltage; a fourth transistor that is digitally controlled and has a first terminal connected to the second terminal of the second transistor and has a second terminal connected to the common voltage; and an actively-controlled, discharge current control circuit connected between a second terminal of the first transistor and a second terminal of the third transistor.
 7. The switch-on control circuit of claim 6, wherein the discharge current control circuit includes a discharge resistor.
 8. The switch-on control circuit of claim 6, wherein the discharge current control circuit includes a mirrored current transistor.
 9. The switch-on control circuit of claim 8, wherein the mirrored current transistor is controlled by a connection between the second terminal of the second transistor and the first terminal of the fourth transistor.
 10. The switch-on control circuit of claim 8, wherein the discharge current control circuit further includes a discharge resistor.
 11. The switch-on control circuit of claim 10, wherein the mirrored current transistor includes a first terminal connected to the second terminal of the first transistor and includes a second terminal connected to the discharge resistor, the discharge resistor being connected between the mirrored current transistor and the third transistor.
 12. The switch-on control circuit of claim 10, wherein the mirrored current transistor is controlled by a connection between the second terminal of the second transistor and the first terminal of the fourth transistor.
 13. The switch-on control circuit of claim 12, wherein the mirrored current transistor includes a first terminal connected to the second terminal of the first transistor and includes a second terminal connected to the discharge resistor, the discharge resistor being connected between the mirrored current transistor and the third transistor.
 14. The switch-on control circuit of claim 6, wherein the first and second transistors comprise p-type MOSFETS, and the third and fourth transistors comprise n-type MOSFETS.
 15. The switch-on control circuit of claim 6, wherein a second terminal of the output capacitor is connected to the common voltage.
 16. An output buffer switch-on control circuit, comprising: a plurality of transistors, at least two including a first and a second transistor being controlled by an output data source; an output capacitor connected to a terminal of at least one of the plurality of transistors; and an actively-controlled, discharge current control circuit disposed between said first and second transistors and including a mirrored current transistor and a discharge resistor, the mirrored current transistor being feedback controlled by the output capacitor.
 17. The switch-on control circuit of claim 16, wherein the output capacitor is connected to a terminal of each of a third and a fourth transistor of the plurality of transistors.
 18. An actively-controlled, discharge current circuit for controlling a discharge current of an output buffer switch-on control circuit, comprising: a mirrored current transistor feedback controlled by an output capacitor of the switch-on control circuit; and a discharge resistor connected to a second terminal of the mirrored current transistor.
 19. The discharge current circuit of claim 18, wherein a first terminal of the mirrored current circuit controls a first transistor of the switch-on control circuit.
 20. The discharge current circuit of claim 19, the discharge current circuit being connected between a pair a transistors each controlled by an output data source, a first transistor of the pair being connected between an internal voltage and the second terminal of the mirrored current transistor, a second transistor of the pair being connected between the resistor and a common voltage, the output capacitor also being connected to a terminal of each of the first transistor of the switch-on control circuit and a fourth transistor that is digitally controlled. 